The invention concerns a method for producing foamed slag on a stainless steel melt in an AOD (argon oxygen decarburization) or MRP (metallurgical refining process) converter or CONARC SSt for stainless steel by adding a foaming material.
During refining in a converter for producing stainless steel, a slag forms which has a high content of metal oxides, especially chromium oxide. The concentration of the chromium oxide during the refining phase often reaches values exceeding 20%, which greatly increases the viscosity of the slag. The slag is usually very hard just before the reduction.
The process metallurgy of these types of slags constitutes the following successively occurring partial reactions:{O2}=2[O] thermal dissociation of the oxygen  (1)2[Cr]+3[O]=(Cr2O3) chromium oxidation in the melt  (2)(Cr2O3)+3[C]=2[Cr]+3{CO} chromium oxide reduction at the slag/metal phase boundary  (3)
Reaction (3) is fundamentally important for the production of stainless steel, because chromium oxide is the most important component in the foamed slag production. However, this reaction takes place to a very limited extent in the conventional process. The reason for this is the high viscosity of the slag. All current converter processes reduce the slag only in the following step, in the so-called reduction, and the process for this is normally carried out with silicon in the form of ferrosilicon.
If, however, the slag viscosity is suitable for foaming, the reduction process (Eq. 3) is made possible by the gaseous CO formed in the refining process itself and by the iron oxide reduction of the foaming material, which is added to the melt in the form of pellets or briquettes at a specific time. The principal components of the material are iron oxide carriers, e.g., scale, then carbon and a ballast material, which defines a systematic placement of the briquette in the melt.
The reduction process in this type of briquette then proceeds by the following principle:(Fe2O3)+3[C]=2[Fe]+3{CO} chromium oxidation in the melt  (4)
The crucial factors for the formation of the foamed slag are thus the constituents of the added foaming material and the slag viscosity, which in turn depends on the composition and the temperature of the molten slag. Above all the viscosity defines a phase region of the molten slag in which foam formation is possible. Therefore, it is important to control the slag basicity, which is responsible for the viscosity, so that the gas bubbles that are formed are forced to stay temporarily in the slag layer. In this regard, the limestone added to control the slag basicity constitutes a further source of gas, since the thermal dissociation of this material releases CO according to the following equation:(CaCO3)=(CaO)+{CO}  (5)
The phenomenon of bubble formation involves a process that utilizes the mechanical force of the reacting gas bubbles to produce a new surface region in the slag. The buoyant forces on the gas bubbles transiently split the slag surface and saturate the whole slag layer to produce the foam. When there is a protracted gas stream from the reacting substances, the number of accumulating bubbles grows with the growing foam. As a result, the height of the foam layer increases with increasing amount of gas; it is directly proportional to the amount of the foaming material.
An important factor in this type of mechanism is the optimal placement of the reactants to obtain a maximum foaming effect. Optimal placement occurs in the boundary region between the slag layer and the liquid metal.
The document JP 1 116018 A describes a method for increasing the efficiency of the blowing process in steelmaking in a converter by adding a mixture, wherein slag formation and foaming is prevented.
DE 195 18 343 A1 describes a method for increasing the effectiveness of the smelting reduction of oxidic metal carriers and improving the thermal efficiency of the charged fuels in the smelting production process.
DE 10 2008 032975 A1 describes a method for producing a foamed slag on stainless steel melts in an electric arc furnace.
Slag foaming has not been used so far in the production of stainless steel. Nowadays only dusts are added as compressed pellets or briquettes for the purpose of realizing cooling effects. Since the reducing element, namely, carbon, and the ballast material are not present in the briquettes, these remain effective only unreduced in the slag zone.
In DE 10 2007 006 529 A1, in the production of a foamed slag on a high-chromium steel melt, the metal oxides, mainly chromium oxide, that are present in the slag are additionally reduced by the briquettes and/or pellets floating near the melt/slag phase boundary, such that the reaction gases that are produced support the foaming of the slag. To this end, the briquettes or pellets charged to the electric arc furnace consist of a well-defined mixture of an iron carrier as the ballast material, carbon or carbon as the reducing agent, and a binder.